Mn—Mo—Ni steel has been known to have excellent strength and toughness, and is mainly used as a material for, for example, a pressure vessel of a nuclear power plant. Such a material has been required to have a toughness level that is increasingly raised from the viewpoint of safety. For example, higher low-temperature toughness is required for a cask used for storage/transport of spent fuel from a nuclear power plant. In addition, higher drop-weight characteristics at low temperature are required for the cask to ensure safety against failure. In step with these, Mn—Mo—Ni-series welding metals used for such applications are also required to be improved in strength, low-temperature toughness, and drop-weight characteristics.
A welded structure including the Mn—Mo—Ni-series welding metal is subjected to long annealing for stress relief after welding (hereinafter, referred to as SR annealing), and carbide is precipitated during the SR annealing, causing variations in characteristics of the welding metal. Hence, there is a need of establishment of a technique for improving strength, low-temperature toughness, and drop-weight characteristics depending on conditions of SR annealing.
For example, Ni-based-alloy welding materials as disclosed in patent literature 1 and 9%—Ni-based-alloy welding materials as disclosed in patent literature 2 are known to be effective for improvement in low-temperature toughness of a welding metal. However, the Ni-based-alloy welding materials are disadvantageous in cost since the materials contain a large amount of expensive Ni. In addition, the 9%—Ni-based-alloy welding materials each have a stable austenite structure formed during SR annealing, causing a significant reduction in yield stress. Hence, there is a need of a technique that improves strength, low-temperature toughness, and drop-weight characteristics of the welding metal while controlling the Ni content at a low level.
On the other hand, for example, patent literature 3 discloses a certain effect of improving low-temperature toughness of a welding metal through formation of a fine acicular-ferrite structure nucleating on Ti-based oxide. In this technique, however, the lowest temperature at which sufficient low-temperature toughness is obtained is still not so low, −60° C. If a larger amount of Ti-based oxide is dispersed for further improvement in low-temperature toughness, coarse Ti oxide, which acts as origin of the fracture, increases. Hence, further devising is required. Patent literature 4 discloses a technique for achieving a welding metal having excellent drop-weight characteristics through controlling flux components and wire components in submerge arc welding. The submerge-arc welding metal, however, has a high oxygen level, leading to formation of coarse oxide. As a result, the lowest no-break performance temperature of the drop-weight characteristics is still not so low, −90° C. Furthermore, patent literature 5 discloses a welding metal having excellent fracture toughness through controlling the Ni content. However, the welding metal also has a high oxygen level, and therefore the low-temperature toughness of the welding metal is considered to be still insufficient.
On the other hand, while patent literature 6 proposes a technique of controlling the content of each of Nb and V of a TIG welding metal as an investigation on a TIG welding metal having a low oxygen content, the added Nb or V has adverse influence on a balance between strength and low-temperature toughness of the welding metal. Hence, the lowest temperature at which a sufficient toughness value is obtained is still not so low, −50° C. Consequently, there is a need of establishment of a novel technique of improving strength, low-temperature toughness, and drop-weight characteristics of the welding metal.